Layered Bonded Structures Formed From Reactive Bonding of Zinc Metal and Zinc Peroxide

ABSTRACT

A system, method, and apparatus for layered bonded structures formed from reactive bonding between zinc metal and zinc peroxide are disclosed herein. In particular, the present disclosure teaches a layered bonded structure wherein two structures are bonded together with a layer including zinc oxide. The zinc oxide is formed through a method that includes processing the two structures by contacting the structures under pressure and applying heat to the structures to promote a reaction with zinc peroxide and zinc metal on one or both of the two structures.

BACKGROUND

The invention is directed to bonding materials using reactive bonding ofzinc peroxide and zinc metal to form zinc oxide.

SUMMARY

The present disclosure relates to a method, system, and apparatus forlayered bonded structures formed from reactive bonding between zincmetal and zinc peroxide. In one or more embodiments, the presentdisclosure teaches a method of forming a bonded layered structure thatincludes providing a first structure and a second structure, forminglayers with zinc metal and zinc peroxide on one or more of the first andsecond structures, processing the first and second structures to causeoxidation of the zinc metal, and forming another layer with zinc oxidebetween the first and second structures. In at least one embodiment, thezinc oxide is formed from oxidation of the zinc metal and deoxidation ofzinc peroxide. In further embodiments, the oxidation results from oxygenprovided by the zinc peroxide, as well as oxygen from ambientconditions.

In one or more embodiments, the present disclosure teaches a bondedlayered structure that includes a first structure and a second structureand a layer comprising zinc oxide between the first and secondstructures, wherein the zinc oxide is formed from oxidation of zincmetal on at least one of the first and second structures. In at leastone embodiment, the zinc oxide is formed from deoxidation of zincperoxide on at least one of the first and second structures.

The features, functions, and advantages can be achieved independently invarious embodiments of the present inventions or may be combined in yetother embodiments.

DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 shows an illustration of a bonding process between two structuresto form a layered bonded structure; and

FIG. 2 shows a method of forming a layered bonded structure.

DESCRIPTION

The methods and apparatus disclosed herein provide a system for bondingof layered structures in applications, particularly applicationsemploying layered wafers, such as in semiconductors and other materialssuch as layered ceramics, layered metals, layered composites.Specifically, this system provides for bonding between layeredstructures by forming a zinc oxide layer that bonds the structurestogether. The zinc oxide results from a reaction occurring between zincmetal and zinc peroxide, wherein the zinc metal and zinc peroxide areprovided in layers on the structures.

Certain processing techniques require bonding of layered materials witha high-transparency and highly conductive medium, such as zinc oxide,between the layers. In previous methods, bonding processes employedmaterials such as: 1) thin metallic interface layers; 2) thin polymerfilm; 3) transparent conducting oxides with direct oxide bondingmechanisms; 4) heavily-doped III-V interface layers; and 5) transparentcarbon nanotube films. Methods employing these materials, however,presented various limitations. For example, the layers had to be highlypolished or required high levels of energy for processing.

To help minimize such limitations in bonding layers together, thepresent method takes advantage of the low melting temperature of zinc toinitiate oxygen bonding in zinc metal and convert the zinc metal intozinc oxide. The oxygen required for zinc oxide formation comes from zincperoxide and, if needed, added oxygen gas. The use of zinc metal allowsflexibility and higher tolerance in surface topography of the hostlayered materials prior to bonding, such that this approach presents ahigh-yield integration scheme.

In the following description, numerous details are set forth in order toprovide a more thorough description of the system. It will be apparent,however, to one skilled in the art, that the disclosed system may bepracticed without these specific details. In the other instances,well-known features have not been described in detail so as not tounnecessarily obscure the system.

The invention is a processing system and method to bond layeredmaterials using transparent conductive zinc oxide (ZnO) produced throughoxidation and reactive bonding of zinc metal (Zn) and zinc peroxide(ZnO₂), resulting in a bonded structure formed from the layeredmaterials and the zinc oxide. FIG. 1 shows a schematic illustration ofthe proposed hybrid zinc oxide bonding approach between a firststructure 10 and a second structure 20. In the illustrated embodiment,the first and second structures 10, 20 provided are first and secondwafers 10, 20. In other embodiments, the first and second structures mayinclude one or both of a particular device or a handle in a layeredunit. In further embodiments, one or both of the structures may bephotovoltaic and/or include a solar cell.

The first and second wafers 10, 20 may include materials such assilicon, crystalline silicate, gallium arsenide, indium phosphide,gallium phosphide, aluminum gallium arsenide, and indium galliumarsenide. Because a layer of zinc peroxide is deposited on the first andsecond wafers 10, 20, as discussed in further detail below, thecompatibility of the materials forming the first and second wafers 10,20 is less impactful, because these materials may not necessarilycontact each other during the bonding process. As such, there is greaterflexibility to include a wider variety of materials in the first andsecond wafers 10, 20.

To further describe the bonding process, material containing zincperoxide is deposited on the first and second wafers 10, 20, forming azinc peroxide layer 30, as shown in the illustrated embodiment. The zincperoxide layer 30 may be transparent and/or conductive. In analternative embodiment, the zinc peroxide layer 30 is deposited on onlyone of the first and second wafers 10, 20. The zinc peroxide layer 30may be deposited under atmospheric pressure and thus does notnecessarily require vacuum systems or other pressure-altering systems,as with other wafer-bonding applications. Also, the zinc peroxide layer30 may be deposited using a plasma-enhanced tool or other tool commonlyused in the field.

Following the application of the zinc peroxide layer 30, a zinc metallayer 40 is then formed on one or both of the first and second wafers10, 20. In the illustrated embodiment, the zinc metal layer 40 isdeposited over the zinc peroxide layer 30 on both of the first andsecond wafers 10, 20. In an alternative embodiment, the zinc metal layer40 may be deposited over one of the first and second wafers 10, 20 thatdoes not have a zinc peroxide layer 30 deposited thereon. The zinc metallayer 40 may have a thickness in the range of 10-100 nanometers (nm) orthicker. As with the zinc peroxide layer 30, the zinc metal layer 40 maybe deposited under atmospheric pressure using a plasma-enhanced tool orother tool commonly used in the field.

After the zinc metal layer 40 is deposited on the first and/or secondwafers 10, 20, the bonding process of the zinc metal with oxygen, i.e.,oxidation of the zinc metal, is initiated. To proceed, pressure isapplied to press the first and second wafers 10, 20 together.Consequently, the zinc peroxide and zinc metal layers 30, 40 are pressedtogether. The amount of pressure applied may be in the range of about 5to 80 psi and can be as high as the host materials 10, 20 can withstand.In addition to applying pressure to the first and second wafers 10, 20,the first and second wafers 10, 20 are exposed to a temperature in therange of about 100-400° C. The temperature should be at a levelsufficient to result in softening of the zinc metal. This may beconsidered a low temperature range in view of other wafer productionmethods, although higher temperatures may be applied. This method,however, takes advantage of the low temperatures at which melting andbonding of zinc metal can occur, such that the bonding process may beconducted at lower temperatures than those associated with othermaterials. This ability to conduct the process at such lower temperatureranges reduces the thermal stress in the bonding material as well asenergy associated with the process.

The application of heat and pressure may occur in a time range of about30 minutes to 3 hours or however long is necessary to achieve thedesired effect. As heat and pressure are applied to the first and secondwafers 10, 20, additional oxygen gas may also be distributed around thefirst and second wafers 10, 20 to promote further oxidation of the zincmetal.

The application of the described heat and pressure results in thereactions shown below:

2 Zn+2 O→ZnO  (1)

2 ZnO₂→2 ZnO+2 O  (2)

Zn+ZnO₂→2 ZnO  (3)

In particular, the application of heat and pressure to the zinc metallayer 40 in the presence of oxygen causes the zinc metal to melt andfurther results in thermal oxidation of the zinc metal to form zincoxide, as seen in equation (1). The oxidation rate may be enhanced byoxygen species production from a thermally-induced ZnO₂-to-ZnOconversion reaction in the zinc peroxide layer 30, as seen in equation(2). A thermodynamic-based binary phase diagram (not shown) of zinc andoxygen also supports a ZnO₂-to-ZnO conversion reaction at the describedtemperature. The net result from the reaction (1) and (2) is zinc oxidebonding, as shown in equation (3). The zinc oxide bonding process may beconducted until most or all of the zinc peroxide has undergone thereaction leading to zinc oxide. The amount of zinc peroxide remainingmay be reduced or eliminated as preferred, depending on the applicationand function of the first and second wafers 10, 20. To verify the extentto which zinc oxide bonding has occurred, the amount of zinc oxidebetween the first and second wafers 10, 20 may be determined bydetection processes known in the art, such as x-ray diffraction andsecondary ion mass spectrometry. Tests may also be conducted forresistance and optical transparency of zinc oxide at specificwavelengths in comparison with known parameters for zinc oxide. The zincoxide bonding process thus results in a layer of zinc oxide 50 that inturn bonds the first and second wafers 10, 20 together, forming aunitary layered bonded structure 60. Note that complete oxidation of Znmetal film may not be necessary in applications that do not require highoptical transparency.

A summary view of the process is shown in FIG. 2. The process is firststarted (step 100), and the first structure 10 and second structure 20are provided (step 110). The zinc peroxide layer 30 is formed on atleast one of the first and second structures 10, 20 (step 120). A zincmetal layer 40 is then formed on at least of the first and secondstructures 10, 20 (step 130). The first and second structures 10, 20 arethen contacted and processed under pressure and heat to cause oxidationof the zinc metal (step 140). A zinc oxide layer 50 is then formedbetween the first and second structures 10, 20 that bonds the first andsecond structures 10, 20 together (step 150). The process may then beended (step 160).

Notably, this bonding approach for zinc oxide conversion relaxes thesurface roughness requirement required by other types of bonding.Specifically, other types of bonding procedures, such as direct waferbonding in which no additional material is layered on the wafers,require that the bonding surfaces are very smooth with minimal surfaceirregularities. This may require a significant amount of production timededicated to treating the bonding surfaces to achieve the desiredreduction of surface irregularities through chemical-mechanicalpolishing and/or other surface treatment steps prior to the bondingprocess. This bonding process may also provide high yield. Inparticular, transparent conductive zinc oxide bonding materials,including zinc peroxide, adhere to many different materials, allowingfor a variety of material combinations for bonding. Further, the bondedzinc oxide provides for increased mechanical strength with reducedelectrical resistance. Zinc oxide also provides optical transparency atcertain wavelengths.

One area in particular for which the present bonding process may beutilized is solar cell technology, or semiconductor-bonded technology,which has produced solar cells with high space cell efficiency. In thistechnology, high cell efficiency depends significantly on the bondquality of gallium arsenide-based solar cells and indium phosphide-basedsolar cells. Since the present process reduces requirements for surfacepreparation and material compatibility inherent in other processes,semiconductor-bonded solar cell designs can be produced with additionaldegrees of freedom, more varieties of materials, and higherefficiencies. For example, on the photovoltaic module level, a solarcell can be bonded to metal or ceramic or light-weight handles. Inaddition, the ability to accommodate various other solar cell materials,such as crystalline silicate, copper indium selenide, and indium galliumnitride, allows for reduced material and integration costs forhigh-efficiency solar cell designs. The described bonding process mayalso be used for any other technology that requires transparentconductive bonding.

Although certain illustrative embodiments and methods have beendisclosed herein, it can be apparent from the foregoing disclosure tothose skilled in the art that variations and modifications of suchembodiments and methods can be made without departing from the truespirit and scope of the art disclosed. Many other examples of the artdisclosed exist, each differing from others in matters of detail only.Accordingly, it is intended that the art disclosed shall be limited onlyto the extent required by the appended claims and the rules andprinciples of applicable law.

We claim:
 1. A method of forming a bonded layered structure, comprising:providing a first structure and a second structure; forming a firstlayer comprising zinc peroxide on at least one of the first and secondstructures; forming a second layer comprising zinc metal on at least oneof the first and second structures; contacting the first and secondstructures such that the first layer contacts the second layer, suchcontact occurring between the first and second structures; processingthe contacted first and second structures to cause oxidation of the zincmetal; and forming between the contacted first and second structures athird layer comprising zinc oxide.
 2. The method of claim 1, whereinprocessing the contacted layers comprises applying heat to the contactedlayers in a temperature range of about 100-400° C.
 3. The method ofclaim 1, wherein processing the contacted layers comprises applyingpressure in the range of about 5 to 80 psi.
 4. The method of claim 1,wherein the zinc oxide is formed from oxidation of the zinc metal. 5.The method of claim 4, wherein the oxidation results from oxygenprovided by the zinc peroxide.
 6. The method of claim 4, wherein theoxidation results from oxygen gas provided to the contacted layers. 7.The method of claim 1, wherein the zinc oxide is formed from athermally-induced conversion reaction of the zinc peroxide.
 8. Themethod of claim 1, wherein at least one of the first and secondstructures comprises a wafer.
 9. The method of claim 1, wherein at leastone of the first and second structures comprises a semiconductor. 10.The method of claim 1, wherein at least one of the first and secondstructures comprises a solar cell.
 11. The method of claim 1, wherein atleast one of the first and second structures comprises gallium arsenide.12. A bonded layered structure, comprising: a first structure and asecond structure; and a layer comprising zinc oxide between the firstand second structures that contacts both the first and secondstructures, wherein the zinc oxide is formed from oxidation of zincmetal on at least one of the first and second structures.
 13. Thelayered structure of claim 12, wherein the zinc oxide is formed fromexposing the first and second structures to a temperature in the rangeof about 100-400° C.
 14. The layered structure of claim 12, wherein thezinc oxide is formed from contacting the first and second structures byapplying pressure in the range of about 5 to 80 psi.
 15. The layeredstructure of claim 12, wherein the zinc oxide is formed from zincperoxide on at least one of the first and second structures.
 16. Thelayered structure of claim 12, wherein the zinc oxide is formed fromoxygen gas provided to the first and second structures.
 17. The layeredstructure of claim 12, wherein at least one of the first and secondstructures comprises a wafer.
 18. The layered structure of claim 12,wherein at least one of the first and second structures comprises asemiconductor.
 19. The layered structure of claim 12, wherein at leastone of the first and second structures comprises a solar cell.
 20. Thelayered structure of claim 12, wherein at least one of the first andsecond structures comprises gallium arsenide.